High-frequency power measuring device



March 11, 1952 M. c. JONES HIGH-FREQUENCY POWER MEASURING DEVICE 2 SI-lEETS-SHEET 1 Filed March 50, 1948 March 11, 1952 M. c. JONES 2,588,390

HIGH-FREQUENCY POWER MEASURING DEVICE Filed March 30, 1948 2 SHEETS-SHEET 2 Patented Mar. 11, 1952 OFFICE HIGH-FREQUENCY POWER MEASURING DEVICE Mack C. Jones, Unionvill e, Qonn Applicatign, March 3!) 1948, Serial No. 173842:

tclai s (01. 171.795,

The present invention use in connection with radio frequency transmission lines, particularly transmission lines such as the open wire and coaxial transmission lines commonly utilized, for example, in radio transmitting and associated circuits. constructed and arranged to be responsive to, incident and reflected voltages at any selected point in the line and is further constructed and arranged to measurethestanding waveratio and the net R. F. power travelingtoward theload at that point.

The device of the present invention has particular utility in the radio field, being particularly useful to permit the operator of a radio station, for. example, to monitor the output of his transmitter and the impedance of his antenna and to allow adjustment of antenna circuits to obtain optimum load conditions, this being particularly advantageous not only for initial adjustment but also to adapt the apparatus to changing conditions. or exa laduetc. erseweath c ng,

etc.

It s an q ie i heinvent qn 9 pr vi ee ceo he c ract e rred to hichis imple touse, whichis accurate andfoolproof in operation, and which is adaptable for use in a wide range of applications such as in line transmitting widely varying frequencies and power.

Another object of the invention is to provide such a device whichwill be efiective for the intended purpose without causing any materialand arrangement of parts which willbe e; ;ernp 1i-;-.

fled in, t e ons ru flmv he e t r ort n he cope of the appli a ion o whic l e nicat d n-the, pp n ed c aim ln. be.d awings E sure-lai a agr mmatic iew: oi on em:

b .n ent hawn. as.

bodiment Fig. 2 1s a imilar rine, mania? relates to a device. for

The device is v w fflase c d embodim t.

third embodiment of the invention; and

Fig. 5 is awiring diagram of a preferred form of meter circuit for use in the device of the in -,7

vention,

Referring first to the embodiment, shownin Fig. 1 of the drawings, the. device of the present,

invention is shown, byway of illustration, con-.-

nected to a coaxial transmission line comprising an inner conductorA and agrounded outer tubue.

lar conductor B. As will" be. apparent, the. in? vention could be applied equally well to other forms of transmission lines. The inner conductor A, in accordance with the invention, is flared outwardly to a larger diameter and separated at I, the flare being provided to maintain continuity of the transmissionline andto minimize reflec- In the embodiment shown in thisfigure aswell as in Figs. 2 and 4',

the left-hand side'of the line (as viewed in the drawing) is assumed to be connected to the trans? mitter while the right-hand side of the line is:

connected to load. A resistance such as the re-.

sistor assembly comprising resistors Zand 2 hav-.. ing a small resistance value compared: with the.

characteristic impedance of the transmission line is utilized to bridge the separated inner conductor A. at I.

Connectedbetween the outer conductor- B and the opposite-ends of innerconductor A aretwo. voltage divider circuits, the first comprising vari-.. able capacitor 3 and fixed capacitor 4, and then other, variable capacitor 3 and fixed capacitor 4'. As will be apparent from the following description, a voltage divider circuit employing resistances of value corresponding to the reactance of capacitorst and 4 and 3'- and 4* maybe substituted for such capacitors, if desired, without departing from the scope of the invention. A rectifier 6 is connected in serieswith symmetrical resistor assembly 2, 2' and capacitor 4 while rectifier 6' is connected in series with resistor assembly 2, 2' and capacitor 4. The

network consisting of. inductance 'l, capacitori inductance Ill and capacitor Il and the network consisting of inductance 1', capacitorif, induce tance l0" and capacitor; II are used. to corn u er, ccnductcrs. AV and: B, as ins.

onduct rs a and-E have. a. ai fcurrehtf ins or pain. tor exam le; at. he

Which is conventional, the lead lines 8 and 8' carry a rectified current proportional to the rectified current of rectifiers 6, while lead lines 9 and 9 carry a rectified current proportional to the rectified current of rectifier ii. The amount of these currents can readily be indicated by the meter circuit shown in Fig. 3 comprising ammeter 23 and potentiometer 34 connected in series to terminals 35 and 36. In effeet, the potentiometer 34 converts the ammeter 23 into a voltmeter. When lead lines 8 and 8' are connected to terminals 35 and 36, the ammeter 23 indicates a rectified current (or voltage) which is in turn proportional to the R. F. voltage impressed on rectifier 6. Similarly, when lead lines 9 and 9 are connected to terminals 35 and 36, the ammeter reading is proportional to the R. F. voltage impressed on rectifierfi'.

The R. F. voltage impressed on rectifierli andhence the ammeter reading when the meter circuit of Fig. 3 is connected to leads 8 and 8 may be made directly proportional to the reflected voltage on the transmission line, and the R. F. voltage impressed on rectifier 6' and hence the ammeter reading when the meter circuit is connected to leads 3, 9 may be made directly proportional to the incident voltage on the transmission line. This follows from the following equations and adjustments:

The voltage drop across the symmetrical resistor assembly 2, 2" is directly proportional to the current in the center conductor A in accordance with the formula (1) E1=IR2 where E1=voltage drop across resistor assembly 2, 2 I=current in center conductor A Rz=resistance of resistor assembly 2, 2

The voltages appearing across capacitor 4 and 4'; respectively, are proportional to the voltage appearing between the inner conductor A and the outer conductor B. For reasons set forth below, the variable capacitors 3 and 3' are adjusted in accordance with the invention to make the voltage across 4 and 4' equal to each other and proportional to the voltage between conductors.A and B in the same ratio as the resistance of resistor assembly 2, 2' is proportional to the characteristic impedance of the transmission line-(assumed to be a pure resistance). This accomplished by satisfying the following mula (2) 65+" cr ii for- The two voltages E1 and E2 are combined and impressed on the rectifier 5 by reason of the connections described above whereby the reading of the meter circuit of Fig. 3 when connected to lead lines 8 and 8' is proportional to the combined voltages as given'by the expression where Es= voltage across rectifier 6. However, .by reason of the connections de scribed above, the voltage impressed on rectifier 5' "is the combination of'voltage E2 afid voltage E,

whereby the reading of the meter circuit when connected to leads 9 and 9' is proportional to the combined voltages as given by the expression where E9=voltage across rectifier B.

By substitution of Formulae 1 and 2 in Formulae 3 and 4 respectively, the following relationship is derived:

It is well known that the voltage on a transmission line can be considered as the sum of the incident and reflected voltages and the current can be considered as the difference between theincident and reflected currents. This relationship may be expressed as follows:

(7) Eb=Ei+Er and (8) IL=Ii-Ir where EL=line voltage 'Er=reflected component of voltage Ei=incident component of voltage Ir.=llne current Ir=reflected component of current I1=incident component of current Therefore, by substituting Formulae 7 and 8 in Formulae 5 and 6, the following relationship is established:

However, it is equally well known that the reflected voltage is equal to the reflected current times characteristic impedance and the incident voltage is equal to the incident current times characteristic impedance. Consequently, Formulae 9 and 10 may be rewritten as follows:

is a constant, E8 is proportional to reflected voltage in the transmission line and E9 is proportional to incident voltage.

If desired, a similar derivation of voltages proportional to reflected and incident voltages respectively may be obtained by use of the circuit Referring to shown in Fig. 2 of the drawings. Fig. 2, it will be seen that the voltage existing between the inner conductor A and the outerconductor B is divided in a pair of similar voltage divider circuits, the .one comprising variable capacitor I3 and resistor [4 while the other comprises variable capacitor l3 and, resistor l4. In this embodiment, the inner conductor A is not separated anda voltage proportional to the current flowing in the conductor 'A'isobtain'ed assassin:

bymutual induotances n: and It. In. other? words, the embodiment shown in 2 com"- prises both lumped and distributed constants unlike the embodiment shown in Fig. l which comprises lumped constants only. If desired,

however, the inner inductor A may be separated as in the case of the embodiment shown in'sFigl 1 i and inductances may be placed intseriesitherewith and the voltage produced across the same may-be substitutedior thevoltage produced in themu'tualinductances l2 and I2.

, The capacitorsl5'and I5 are usedto counter- I act stray fields in the unit which might cause an i Since this induced voltage is proportional to ire-- quency, its efiect can be cancelled another-.-

equal and opposite. eirect. whichalso. is. proportional to frequency by means of the small capacitors I5 and I5;

The resistors I4 and I4 are connected' in series with the mutual induct'ances l2 and IZ 'respectively and the current therein is rectified by rectifiers I6 and I6" respectively. The voltage derived at terminals'lt and I8, therefore, is proportional to the rectified current flowing through rectifier I 6 while the voltage across terminals 19 and I9 is proportional to the rectified. current of rectifier I6. Capacitors 2| and 2| are. utilized to isolate these circuits from extraneous effects caused by the voltage between the inner and outer conductors.

The voltage drop across the resistors I 4 and I4 is obtained by current flowing. through the capacitors l3 and. I3 respectively. By utilizing capacitors having a high reactance compared with the resistance of the resistors I4 and. I4, the-voltageacrcss these resistors may be taken to be substantially as follows:

where E14=voltage component across resistor I 4. or I4 E=line voltage R1 =resistance of resistor I4 on I4 7 X=impedance of capacitor I3 or I3 W=21r timesfrequency C=capacitance-of capacitors [3 or I3 The voltage produced across the mutual inductances I-2 or l2 may be expressed asfollows:

( 14)- E12=IWMC where Eiz=voltage across mutual inductances I2 or I2 I=line current W=27r times frequency M=inductance I2 or I2 These two voltages are then combined and rectified by the rectifiers I6 and I61. Asaresult, the voltages appearing at thetermi'nals l8 and I8 and terminals I9 and I9" may be given by the- By suitable; adjustment I of z the. capacitors: It. and: 1 3f the iollowingrelationship smay bez satis' fied:

(17)" RoR2C=M' Rewriting the expressions'ior Em and. Era, we." nowhave:

where B=a:constantz This will be seen to-besimilar to the equations for E8 and E9 obtained with the coupler circuit shown in Fig. 1 of the drawings as described above. There is, however; one important differ-- -20; ;ence', namely, that the voltages obtained by the circuit: shown in Fig. 2 aredirectly proportional tothe frequency of measurement. This means that the circuit as shown is sensitive to frequency and its power scales must be calibrated for each 251individual frequency used. This is not true,

however, when the standing wave ratio is measured'inasmuch as this includes acalibration pro cedure as described hereinafter.

By modifying the circuit shown in Fig; 2 to aoz-lobtain the circuit shown in Fig. 4 of the draw ings, this dependency on frequency may be elimi nated and the instrument may be calibrated exactly as the instrument shown in Fig. 1. In the circuit shown in Fig. 4, the variable capacisaitors 23 and 23' correspond to the variable capacitors I3 and I3; resistors 24 and 24' correspond to the resistors I4 and I4; mutual inductances 22 and 22 correspond to mutual inductances I2 and I2; reotifiers 26 and 26' correspond to rectiifiers I6 and I6, and capacitors 3| and 3| correspond to capacitors 2i and 2t; In the circuit shown in Fig. 4, the essential modification comprises the addition of capacitance by'means of the capacitors 21 and 21. These constants are w selected so that the reactance of the capacitors is small compared with the value of resistance. Resistors 25 and 25' are added, if necessary, to supplement resistors 24 and 24'. As a result, the voltages at terminals 28 and 28 and 29 and 29 ,50tf(Formulae18 and 19) are then modified as to]:

Ezs=voltage across lines 28, 28 Emi -voltage across lines 29, 29'

D=a constant Xe=impedance of capacitor 21 or 21' R=resistance of resistor 25 or 25' These expressions for E28 and E29 are now identical with those from the circuit showninFig: 1 except for the constant. The constant, of course. may be taken careofby suitablecalibration.

For simplicity of presentation, the embodima-ments shown in Figs. 1, 2 and 4 comprise symr metrical circuits, one for deriving a voltage com.- ponent proportional to incident voltage. and the; other for deriving a voltage component 131301101? tional to reflected voltage. Inasmuch as: these circuits-are-identical except as to the: way they;

7 are connected to the line it will be obvious that a single circuit may be used for both purposes merely by providing suitable switching means for connecting it to the circuits denoted by unprimed reference numerals or to the circuits denoted by primed reference numerals. as desired.

As previously mentioned, the meter circuit of Fig. 3 which, in essence, is a volt meter, is utilized for direct measurement of voltage values which are proportional to reflected and incident voltage respectively. However, in accordance with the invention, it is desired to provide means for utilizing such measurement to show the standing wave ratio and/or net power traveling down the line. This is accomplished in accordance with the invention by the meter circuit shown in Fig. 5 of the, drawings. Referring to Fig. 5, the circuit comprises an ammeter 43 which is grounded at one end and connected at the other end to a plurality of resistors 33, 40 and 4|, the latter resistor being a variable resistor or potentiometen. A multiple contact switch 38 is provided to make connections selectively with any one of these resistors individually. Switch 33 is connected to switch 31 which permits contact with either terminal 4| or 44. Terminal 4| is adapted to be connected to one of the leads 8, 8'; I8, l8; or 28, 28 depending on which of the previous circuits is utilized. Terminal 44 is adapted to be connected to leads 9, 9'; I9, I9 or 29, 29' in a similar manner. Terminals 42 and 45 are grounded so as to be connected to the opposite side of ammeter 43.

By manipulation of the switch 31, the meter can be connected to either the incident or reflected voltage measuring circuits, as desired. To read standing wave ratio, the switch 38 is put in the position shown in the drawings, i. e., connected with variable resistor 4|, and switch 31 is set to connect the switch 38 with terminal 44. The meter 43 is then set to just full scale by adjusting the variable resistor 4|. This is equivalent to taking the incident voltage as unity for reference. The switch 31 is then shifted to connect the circuit to terminal 4| and the meter 43. now reads the reflected voltage as a percent of the incident voltage. This last reading, therefore, gives the ratio between E8 (or E18 or E28) and E9 (or E19 or E29) or the ratio between the reflected and incident voltages. This is equal to the reflection coefficient (K) of the load reflected to the point in the transmission line where the measurement is being made. I

It is well known that the standing wave ratio is related to the reflection coefiicient (K) by the following formulai 7 Since the meter scale reads directly the reflection coefficient, it may be calibrated in accordance with the above expression so that the standing wave ratio may be read directly from the scale. The power standing wave ratio is, of course, the square of the voltage standing ratio.

To obtain power readings, the following procedure is followed: The switch 38 is connected to an appropriate value of resistance, i. e., either the resistor 39 or 40, so that the meter is on scale. The switch 31 is then connected either to the incident or reflected voltage circuits. The values of resistors 39 and 40 are selected so that the meter reading may be interpreted in terms of actual transmission line voltage. With the meter 43' in position to read the-refiected voltaga-the 22 SWR:

45 'vided, in accordance with the invention, a measintended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not used in the following claims is intended to cover reflected power can then be obtained from the following formula:

where Wr=reflected power Er=reflected voltage Ex voltage of lines 8, 8'; l8, l8; or 28, 28

Similarly, with the meter 43 connected to terminals 44 .and 45 so that it reads the incident voltage, the incident power can be obtained from the formula:

Wi=incident power Ez=line voltage 'Ey=voltage of lines 9, '9'; l9, l9; or 29, 29'.

However, inasmuch as the ratio of R0 to 4R2 is a constant, the foregoing Formulae 23 and 24 may be rewritten as follows:

(25) WTZAEIZ (26) W1=AE where O A-a constant- T522 It thus may be seen that both reflected and incident power may be read on the same meter scale simply by connecting switch 31 either to terminal 4| or 44. The meter scale may be calibrated in accordance with the above formula.

(27) net power=Wz-Wr It thus will be seen that there has been prouring device having great usefulness, particularly in the radio transmission field, which device is simple and efficient in operation and which greatly facilitates the proper adjustment of the load.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is in a limiting sense.

It is also to be understood that the language all of the generic and specific features of the invention herein described and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

I claim as my invention:

1. In a, device for measuring standing waveratio and power in a two-conductor frequency transmission line comprising a mutual induct: ance for developing a first voltage component proportional to line current and'frequency, a

voltage divider for developing a. second voltage component proportional to line voltage and frequency comprising a capacitor and a resistor Whose values are such that the product of the capacitance and resistance of said capacitor and resistor, respectively, and the characteristic impedance of the line is equal to said mutual inductance, means for connecting the voltage divider across the two conductors, a circuit comprising said mutual inductance and said resistor in series and having an output voltage equal to the sum of said voltage components which varies directly with frequency, and a load on the said circuit comprising a capacitor of relatively low reactance compared with the resistance of said resistor, whereby said output voltage is rendered independent of frequency.

2. In a device for measuring standing wave ratio and power in a two-conductor frequency transmission line comprising a mutual inductance for developing a first voltage component proportional to line current and frequency, a voltage divider for developing a second voltage component proportional to line voltage and fre quency comprising a capacitor and a resistor whose values are such that the product of the capacitance and resistance of said capacitor and resistor, respectively, and the characteristic im-.

pedance of the line is equal to said mutual inductance, means for connecting the voltage divider across the two conductors, a circuit comprising said mutual inductance and said resistor in series and having an output voltage equal to the sum of said voltage components which varies directly with frequency, and a load on the said circuit comprising a voltage divider having an output voltage to input voltage ratio reversely proportional to frequency, whereby a resulting voltage is produced independent of frequency.

3. In a, device for measuring standing wave ratio and power in a two-conductor frequency transmission line comprising a, resistor for developing a first voltage component proportional to line current and independent of frequency, a voltage divider comprising a pair of capacitors whose reactances are such that the ratio of the capacitors is equal to the ratio of the resistor to the characteristic impedance of the transmission line, means for connecting the voltage divider across the two conductors to develop a voltage component proportional to line voltage and independent of frequency, and means for combining said two voltage components.

MACK C. JONES.

REFERENCES CITED The following references are of record in the file of this patent:

Monitoring Power and Impedance at High Frequencies, by Morrison and Younker, presented 1947 I. R. E. Convention, New York, New York, pages 1 through 5. Copy in Division 65, U. S Patent Oflice. 

